Microlens array

ABSTRACT

A method of forming a microlens including, in one aspect, depositing a substantially non-photo-imageable microlens material over an area of a chip, a portion of which contains an array of photosensitive circuits, and patterning the microlens material over the array of photosensitive circuits to define a microlens over each photosensitive circuit.

BACKGROUND

The invention relates to optical devices and more particularly to amicrolens structure or array over a photosensitive array.

Description of Related Art

Digital imaging systems, such as, for example, digital cameras, utilizeintegrated circuit devices or chips as image capturing devices. Animaging system, such as a camera, uses light to capture an image on asemiconductor-based chip. The chip replaces film in traditionalfilm-based systems. In a digital camera, an image sensor is configured,in its simplest form, to capture a monochrome or color image by way offield effect transistors (FETs), such as complementary metal oxidesemiconductor (CMOS) devices or charge coupled devices (CCDs).

In one example, the image sensor comprises a semiconductor chip made upof a number of photosensitive circuits, each photosensitive circuitcapable of absorbing light. In color applications, each photosensitivecircuit generally absorbs light through a color filter that represents aparticular wavelength of light in the visible spectrum corresponding tothe image sensed.

The photosensitive circuits of an image sensor, often referred to aspixel circuits, are generally arranged in an array, such as for example480 rows by 640 columns. In general, each photosensitive circuitcontains a photosensing element, such as a photodiode or charge-coupleddevice (CCD), and other circuitry. The photosensing element defines aphotosensing region or area of the photosensitive circuit that respondsto light while the circuitry, generally speaking, drives a light signalfrom the photosensing region or area to other process circuitry.

One method of converting a monochromatic digital imaging system into acolor imaging system involves absorbing light through a color filter.The color performance of any color filter concerns the ability of thefilter to select color corresponding to the desired wavelength of thevisible spectrum of the color filter array (CFA) material. A commoncolor filter material is spin coated-, dyed-, or pigmented-photoresistCFA material.

In order to improve the light collecting efficiency of a photosensingcircuit, a microlens is typically formed on top of the CFA materialoverlying each photosensitive circuit. A planarization layer having hightransparency properties may also be deposited between the color filterarray and the microlens material.

The microlens material is typically a photoresist. Initially, even thegenerally transparent photoresist is initially yellow or otherwise notentirely transparent after formation. The lack of transparency isgenerally attributed to the photosensitivity component (such as aphotoacid compound) of the photoresist material. In order to increasethe transparency, the photoresist is often bleached. Photobleachingoccurs after the deposition and patterning of the microlens material,for example, as a photobleaching with ultraviolet light to cross-linkthe photoresist molecules and destroy the photosensitivity of themicrolens. In general, the greater the cross-linking and the destructionof the photosensitivity of the microlens, the greater the transparencyclarity of the microlens. An incomplete photobleaching will result in amicrolens that is not completely transparent or that is yellow.

Another problem with using photoresist material as the microlensmaterial is that temperatures greater than 150° C. tend to degrade thephotoresist material and cause the deformation of the microlens shape.Many steps in forming an image sensor, however, sometimes includeheating to greater than 200° C., such as surface mount processes tocouple the sensor package to a printed circuit board (PCB). Suchtemperatures therefore may damage or destroy the microlenses and thebenefits desired with the incorporation of the microlenses. Thus, theinstability at high temperatures of photoresist-based microlens materialhas generally restricted image sensor packaging on printed circuitboards to manual processes such as local heating solder processes inwhich package leads are connected through soldering while the sensorwith CFA material and microlens material is kept at a lower temperature.

What is needed is an improved microlens.

SUMMARY

A method of forming a microlens is disclosed. In one aspect, the methodincludes depositing a substantially non-photo-imageable microlensmaterial over an area of an integrated circuit chip, a portion of whichcontains an array of photosensitive circuits, and patterning themicrolens material over the array of photosensitive circuits to define amicrolens over each photosensitive circuit. A photosensitive array andan imaging system are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an imaging system incorporating an imagesensor of an embodiment of the invention.

FIG. 2 shows a diagrammatical view of a portion of the imaging system ofFIG. 1.

FIG. 3a shows a schematic, cross-sectional side view of an image sensorthat may be used in an embodiment of the invention.

FIG. 3b shows a schematic perspective top view of the sensor of FIG. 3a.

FIG. 4 shows a cross-sectional side view of a portion of an image sensorincluding three photosensitive circuits and a bond pad, a transparentpassivation layer overlying the photosensitive circuits, CFA materialoverlying the passivation layer, and a planarized microlens materialoverlying the CFA material in accordance with an embodiment of theinvention.

FIG. 5 shows the sensor of FIG. 4 after further processing by depositinga photoresist masking layer over the planarized microlens material inaccordance with an embodiment of the invention.

FIG. 6 shows the substrate of FIG. 4 after further processing byinitially patterning the photoresist masking layer in accordance with anembodiment of the invention.

FIG. 7 shows the substrate of the sensor of FIG. 4 after furtherprocessing by shaping the photoresist masking material to form amicrolens curvature in accordance with an embodiment of the invention.

FIG. 8 shows the sensor of FIG. 4 after further processing by etchingthe microlens material using the photoresist mask as a pattern to formthe microlens with the desired curvature and to open a bond pad inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

The invention relates to an optical device, such as an image sensor tobe used, for example, in a camera application. One aspect of theinvention involves the method of forming a microlens of anon-photo-imageable material on an image sensor of an integrated circuitchip, such as a CMOS- or CCD-based image sensor. The advantage of usinga non-photo-imageable material is the concerns of incompletecross-linking and yellowing of photosensitive resist may be reduced.Further, the concern of a photo-imageable material, such as aphotoresist, degrading under the high temperature condition typicallyused in sensor fabrication is reduced. Embodiments of the inventionrelate to an image sensor, a method of forming a microlens, and animaging system, such as a camera, that incorporate an image sensorhaving microlenses such as described above.

FIG. 1 illustrates an embodiment of an imaging system. Imaging system100 includes optical system 130 that channels the incident energy, e.g.,light, to create an optical image on image sensor unit or image sensingunit 105. Control signal generation logic 118 is provided to generatereset signals in word lines employed to control photosensitive circuitsof image sensor unit 105. Output values (e.g., sensor signals) may beprocessed in analog form before being fed to an analog-to-digital (A/D)conversion unit 110 that in turn feeds digital processing block 114. A/Dunit 110, and portions of the digital processing block 114 may belocated on the same die or chip as the photosensitive circuits althoughthe invention is not limited in this respect. Digital processing block114 may include hard-wired logic and/or a programmed processor thatperforms a variety of digital functions, including preparing digitalimage data based on the sensor signals for storage or transmission.

Transmission of the image data to an external processing system may beaccomplished using communication interface 124. For instance, as adigital camera, system 100 will contain a communication interface thatimplements a computer peripheral bus standard such as, for example,universal serial bus (USB) or IEEE 1394-1995 although the invention isnot limited in this respect. Imaging system 100 may also contain localstorage 128 of the non-volatile variety including, for instance, a solidstate memory such as a removable memory card, a rotating magneticdevice, or other suitable memory device for permanent storage of digitalimage data. The operation of system 100 may be orchestrated by systemcontroller 122 that may include a conventional microcontrollerresponding to instructions stored as firmware.

FIG. 2 shows a diagrammatical view of an embodiment of a portion of adigital imaging system including optical system 130 and image sensorunit 105. Image sensor unit 105 includes image sensor 175 in package172. Conventional materials for package 172 include, but are not limitedto, ceramic and plastic. In one embodiment, the top surface of package172 includes a transparent cover substrate 171, such as glass orplastic, that overlies image sensor 175.

Overlying image sensor unit 105 and referenced as optical system 130 islens assembly 173 and infrared blocking filter 174. Infrared blockingfilter 174 overlies lens assembly 173 and serves to inhibit infraredlight from striking image sensor unit 105.

In the insert of FIG. 2 is a magnified view of image sensor 175. Amagnified view of image sensor 175 in turn shows some additional sensorcomponents. Included within the components shown in FIG. 2 is theuppermost metal layer 177 having illustrative openings intended tomodulate photosensitive circuit or pixel circuit responsivity. Overlyingmetal layer 177 is an array or mosaic of color filter array (CFA)material 178 covering, in this instance, four different photosensitivecircuits or pixel circuits of image sensor 175: One Red, two Green, andone Blue. The array or mosaic represents an illustrative tiling patternfor CFA material 178.

In one example, image sensor 175 comprises a chip made up of a number ofphotosensitive circuits, each photosensitive circuit capable ofabsorbing light. FIG. 3a illustrates a schematic, cross-sectional sideview of a portion of image sensor 175. FIG. 3b illustrates a topperspective view of image sensor 175. Image sensor 175 is fabricated, inthis embodiment, as part of a die or wafer 10 with a plurality of otherdevices. Once formed, individual image sensors are separated from oneanother typically by a sawing process. The individual image sensors arethen placed in a package, such as package 172, of image sensor unit 105as described above.

In FIGS. 3a and 3b, the photosensitive element occupies a region or areaof image sensor 175 illustratively represented by photosensitive area210. In addition to photosensitive area 210, image sensor 175 containsadditional logic circuitry that operates the individual photosensitivecircuits and drives signals from the pixels off image sensor 175. InFIGS. 3a and 3b, the logic circuitry occupies an area of image sensor175 illustratively represented by logic area 220. It is to beappreciated that logic circuitry is not or need not be limited to logicarea 220. Logic area 220 typically represents an area around theperiphery of the sensor of logic circuitry as opposed to photosensitivestructures, like, for example, photodiodes.

To provide power to image sensor 175 and to drive signals on and offimage sensor 175, image sensor 175 contains bond pads 225. Bond pads 225are generally arranged on the periphery of image sensor 175 and surroundphotosensitive area 210 and logic area 220. Bond pads 225 are typicallylocated on the extreme periphery of image sensor 175 in contact or bondpad area 240. Bond pads 225 are electrically linked or coupled to devicecircuitry 222 that may include logic circuitry. Logic area 220 andcontact or bond pad area 240 collectively define non-photosensitive area245, separate from photosensitive area 210.

Overlying the top surface of image sensor 175 is transparent passivationlayer 230. Passivation layer 230 is, for example, silicon nitride (Si₃N₄) or silicon oxynitride (Si_(x) N_(y) O_(z)). Si₃ N₄ and Si_(x) N_(y)O_(z) are chosen because of their transparent properties and theirparticular resistance to environmental contaminants, particularlymoisture. Passivation layer 230 is deposited to a suitable thickness,such as, for example, approximately 8,000 angstroms (Å) according tostate of the art technology. Passivation layer 230 overlies the entiretyof image sensor 175, including photosensitive area 210 andnon-photosensitive area 245 (logic area 220 and contact or bond pad area240).

Overlying passivation layer 230, particularly in photosensitive area210, is CFA material 295 such as dyed- or pigmented-photoresist. CFAmaterial 295 is patterned into an array of color filter channels, onechannel typically above one photosensitive circuit or pixel circuit. Thecolor channels selectively allow light corresponding to a predeterminedrange of the visible spectrum to pass through a channel to image sensor175. The group of color filter channels (e.g., Red, Green, Blue) make upa color system that either alone or by a mathematical manipulation,match or predict the human eye response.

Overlying CFA material 295, particularly in photosensitive area 210, ismicrolens material 300. Microlens material 300 serves, in one aspect, toincrease the light collecting efficiency of each photosensitive circuit.The photosensitive element of each photosensitive circuit is typicallysurrounded by layers of conductive (e.g., interconnection lines) andnon-conductive (e.g., insulating dielectric) layers creating a valleyeffect in which the photosensitive element, such as a photodiode, is atthe base of the valley. A portion of light directed at an angle towardthe photosensitive element can be obstructed from striking thephotosensitive element by the surrounding layers. Microlens material 300overlying the photosensitive circuit serves to redirect the otherwiseobstructed light at the photosensitive element. Thus, the final shape ofmicrolens material 300 generally determines the amount of light directedat the photosensitive circuit.

FIG. 4 illustrates a cross-sectional side view of a portion image sensor175. FIG. 4 shows a portion of photosensitive area 210 including, inthis example, three photosensitive circuits 215. Overlyingphotosensitive circuits 215 is passivation layer 230 of, for example,Si₃ N₄ and Si_(x) N_(y) O_(z). Overlying each photosensitive circuit 215in photosensitive area 210 is CFA material 295. Finally, FIG. 4 showsmicrolens material 300 overlying photosensitive area 210.

In one embodiment of the invention, microlens material 300 is a highlytransparent polymeric coating. Microlens material 300 includes materialsthat have a high transmissivity (>90%) across the visible spectrum oflight (380-780 nm). The material should be stable in the presence ofconventional processing conditions, including temperatures for imagesensor fabrication and packaging. For example, microlens material 300should be stable for temperatures in excess of 200° C. and should notdegrade or yellow in the presence of temperature or other environmentalfactors including moisture uptake. Examples of suitable materials fornon-photosensitive, thermally stable microlens material 300 include, butare not limited to, acrylic polymers with cross-linking components suchas certain hydroxyl, epoxy, and amino compounds that may cross-link withone another, silicones, particularly organosilicons, and polysiloxanes.Suitable materials also include substantially colorless polyimide andperfluorocyclobutane containing ether polymers.

The transparent, thermally stable, non-photosensitive microlens materialcan typically be obtained from commercially available sources. Anexample of transparent acrylic overcoat microlens material is XP-9480from Shipley Corporation, Marlboro, Mass. 01752. The material is basedon an acrylic composition with cross-linking components that consist ofhydroxyl groups and amino cross-linkers. A detailed chemical compositionis not available due to its proprietary nature. The transmittance at 400nm is 99.7% and 100% in 550 nm after curing for 45 minutes at 250° C.The material has been used as a transparent overcoat for flat paneldisplay applications as revealed by the materials supplier.

Another example of a thermally stable, transparent microlens material iscolorless polyimide. An example of such colorless polyimide is ahexafluordihydride diphenyldisulphone formed polyimide with chemicalstructure as follows: ##STR1##

The colorless polyimide material is commercially available under thetrade name of LaRC from SRS Technologies, System Technology Group, 500Discovery Drive, Huntsville, Ala. 35806-2810. The polyimide can be spunthrough spin coating of a stoichiometrical mixture of2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HFDA) and3,3'-diaminodiphenylsulphone (DDSO₂) in solvent dimethylacetamide andcured in a forced air oven at 150° C. and 250° C. each for one hour. Thematerial has a transmittance larger than 95% in a wavelength range of400-700 nm. The glass transition temperature of this material isapproximately 279° C. which is much higher than typical packagesoldering temperatures which are around 210 to 230° C. The glasstransition temperature is generally the temperature where the materialschange from a stable glassy state to a more deformable softening state.More information about this material can be found from the publishedpaper titled "Evaluation of Colorless Polyimide Film for Thermal ControlCoating Applications," by Anne K. St. Clair, and Wayne S. Slemp, NASATechnical Memorandum 86341, National Aeronautics and SpaceAdministration, Langley Research Center, Hampton, Va. 23665.

A further example of a thermally stable transparent material suitable asa microlens material is perfluorocyclobutane containing aromatic etherpolymer with a chemical structure as follows: ##STR2##

The material is available under trade name of XU-35033.00 from DowChemical, Midland, Mich. 48674. A thin film microlens material is formedby first coating a prepolymer solution containing1,1,1-tris(4,trifuorivinyloxyphenylethane) in solvent mesityleneavailable under the trade name of XU-35031.00, then heating the spunfilm at 180° C. for 1 hour. The prepolymer will undergo a thermalcyclization reaction to form thermal stable cured film. A final glasstransition of 400° C. can be obtained.

The material of microlens material 300 is non-photo-imageable. As notedabove, photo-imageable materials, such as photoresist, tend to discolor(e.g., yellow), particularly where the cross-linking effected by a lightsource (e.g., photobleaching) is incomplete. It is to be appreciated,however, that photo-imageable material is suitable for microlensmaterial 300 provided the cross-linking is complete and the material isstable at fabrication temperatures.

Microlens material 300 is deposited to a thickness generally governed bythe desired final lens size and shape. A suitable thickness for currenttechnology is approximately 1.0 μm-3.0 μm. One factor in determining thedesired thickness of microlens material 300 is a desired microlenscurvature or sag of, for example, a convex microlens. The formation of acurved microlens will be described in detail below. However, for currentpurposes, a suitable curvature or sag, measured from the apex of curvedmicrolens material 300 to the CFA material, would be, for example,approximately 1.0-1.5 μm.

FIG. 5 shows the sensor of FIG. 4 after further processing by spinningon a positive photoresist masking material 320 over the substrate.Photoresist masking material 320 is spun on, in this embodiment, to forma mask to define a microlens area. Photoresist masking material 320, inthis example, is photo-imageable. Photoresist masking material 320 isexposed to a light source through a reticle to define a mask pattern forphotoresist masking material 320 over each photosensitive circuit 215and exposing microlens material 300 directly overlying bond pad 310. Thelight source causes exposed photoresist masking material 320 tocross-link and polymerize. Once the pattern is established, theunexposed photoresist masking material 320 is rinsed away with adeveloper to leave a mask pattern as shown in FIG. 6. The substrate isbaked, according to conventional methods, to solidify the remainingphotoresist masking material 320. It is to be appreciated that there areother suitable materials, beyond positive photoresist, that are suitableas a masking layer in an embodiment of a method of the invention.

Once the mask pattern of photoresist masking material 320 is defined,the sensor is heated to approximately 150° C. or other suitabletemperature to cause photoresist masking material 320 to melt. Thesurface tension resulting from the melting process causes photoresistmasking material 320 patterned individually over each photosensitivecircuit 215 to adopt a curve or arcuate shape, referred to as a sagpattern. The sag pattern shown in FIG. 7 will be used as an initialmicrolens shape to form microlenses in microlens material 300 having asimilar pattern.

Once the initial sag pattern is formed, microlens material 300 ispatterned using photoresist masking material 320 as an initial pattern.A suitable patterning method includes a reactive ion etching ofmicrolens material 300. Suitable etch chemistries and etch rates forcarrying out the etching process are determined based at least in parton the nature of the material for microlens material 300 and the desiredshape of the final microlens. For example, for microlens material 300 ofan acrylic polymer such as polymethylmethacrylate, a suitable etchantincludes an O₂ /CF₄ etch chemistry with O₂ concentration greater than90%.

FIG. 8 shows the image sensor of an embodiment of the invention afterreactive ion etching to form a microlens pattern of microlens material300. As shown in FIG. 8, microlens material 300 adopts, at leastinitially, the lens shape or sag shape of photoresist masking material320. Thus, the thickness of photoresist masking material 320 and itsmelting properties at least in part determine a final microlens shapefor planarization material 300. A second variable in this embodiment ofthe invention to establish a desired microlens shape is the etchchemistry and the etch rate used to define the microlenses over eachphotosensitive circuit. By choosing the appropriate etch chemistry andetch rate, the microlens shape may be patterned as desired.

The above description of an embodiment the invention describes a processof utilizing a microlens or planarization material other thanspecialized photoresist to form a microlens array for the image sensor.In this manner, materials may be chosen for the microlens material thathave chemical and physical properties that are resistant to degradationin the presence of increased temperature or environmental effects. Inthis manner, microlenses may be formed that do not suffer the problemsassociated with prior art specialized photoresist microlenses includingyellowing and other degradation. Further, since a material may be chosenfor the microlens material that is highly transparent, photobleachingthe microlens material can be eliminated.

This embodiment of a process of the invention also improves the controlof the microlens curvature. This embodiment of the invention describesthe formation of the microlenses, at least in part, by etching. Bycontrolling the etch activity of the microlens or planarization materialin addition to the initial microlens photoresist pattern, the curvatureof the final microlens shape can be increased or decreased from theinitial microlens shape created by melting the photoresist.

In the preceding detailed description, the invention is described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the claims. The specification and drawings are, accordingly, tobe regarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method of forming a microlenscomprising:depositing a substantially non-photo-imageable microlensmaterial over an area of an integrated circuit chip a portion of whichcontains an array of photosensitive circuits; and patterning themicrolens material over the array of photosensitive circuits to define amicrolens over each photosensitive circuit.
 2. The method of claim 1,wherein prior to depositing a microlens material, the method furthercomprises:patterning a color filter material over each photosensitivecircuit.
 3. The method of claim 1, wherein prior to patterning amicrolens material, the method further comprises:patterning a maskingmaterial over the microlens material substantially corresponding to theunderlying array of photosensitive circuits.
 4. The method of claim 3,wherein patterning the masking material further comprises patterning thetop surface of the masking material to have a substantially convexshape.
 5. A method of forming a microlens comprising:patterning a colorfilter material over each of an array of photosensitive circuits of anintegrated circuit chip; depositing a substantially non-photo-imageablemicrolens material over the color filter material; and patterning themicrolens material over the color filter material to define a microlensover each photosensitive circuit.
 6. The method of claim 5, whereinprior to patterning a microlens material, the method furthercomprises:patterning a masking material over the microlens materialsubstantially corresponding to the underlying array of photosensitivecircuits.
 7. The method of claim 6, wherein patterning the maskingmaterial further comprises patterning the top surface of the maskingmaterial to have a substantially convex shape.
 8. The method of claim 6,wherein depositing a microlens material further comprises depositing themicrolens material over an area of a chip a portion of which contains atleast one bond pad, and wherein patterning the masking material furthercomprises patterning the masking material to expose the microlensmaterial overlying the at least one bond pad.
 9. An apparatuscomprising:an array of photosensitive circuits on a substrate; an arrayof color filter material over the array of photosensitive circuits, oneof the array of color filter material being over one of the array ofphotosensitive circuits; and a substantially non-photo-imageablemicrolens material over each one of the array of color filter material.10. The apparatus of claim 9, wherein one surface of the microlensmaterial has a generally convex shape.
 11. The apparatus of claim 9,wherein the microlens material comprises one of a cross-linked acrylicpolymer and a cross-linked polysiloxane.
 12. The apparatus of claim 9,wherein the microlens material comprises perfluorocyclobutane etherpolymer.
 13. The apparatus of claim 9, wherein the microlens materialcomprises a substantially colorless polyimide.
 14. An imaging systemcomprising:an imaging sensor of an integrated circuit having an array ofphotosensitive circuits on a substrate capable of providing sensorsignals in response to incident light and control signals, an array ofcolor filter material over the array of photosensitive circuits, one ofthe array of color filter material over one of the array ofphotosensitive circuits, and a substantially non-photo-imageablemicrolens material over each one of the array of color filter material;control circuitry configured to generate control signals for controllingthe imaging sensor; and signal processing circuitry to generate imagedata in response to imaging sensor signals.
 15. The photosensitive arrayof claim 14, wherein one surface of the microlens material has agenerally convex shape.
 16. The photosensitive array of claim 14,wherein the microlens material comprises one of an acrylic polymer and apolysiloxane.
 17. The photosensitive array of claim 14, wherein themicrolens material comprises fluorocyclobutane ether polymer.
 18. Thephotosensitive array of claim 14, wherein the microlens materialcomprises a substantially colorless polyimide.